recognition part ii
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Recognition Part II. Ali Farhadi CSE 455. Face recognition: once you’ve detected and cropped a face, try to recognize it. Detection. Recognition. “Sally”. Face recognition: overview. Typical scenario: few examples per face, identify or verify test example - PowerPoint PPT PresentationTRANSCRIPT
RecognitionPart II
Ali FarhadiCSE 455
Face recognition: once you’ve detected and cropped a face, try to recognize it
Detection Recognition “Sally”
Face recognition: overview• Typical scenario: few examples per face,
identify or verify test example• What’s hard: changes in expression,
lighting, age, occlusion, viewpoint• Basic approaches (all nearest neighbor)
1. Project into a new subspace (or kernel space) (e.g., “Eigenfaces”=PCA)
2. Measure face features
Typical face recognition scenarios
• Verification: a person is claiming a particular identity; verify whether that is true– E.g., security
• Closed-world identification: assign a face to one person from among a known set
• General identification: assign a face to a known person or to “unknown”
What makes face recognition hard?
Expression
What makes face recognition hard?
Lighting
What makes face recognition hard?
Occlusion
What makes face recognition hard?Viewpoint
Simple idea for face recognition1. Treat face image as a vector of intensities
2. Recognize face by nearest neighbor in database
x
nyy ...1
xy kk
k argmin
The space of all face images• When viewed as vectors of pixel values, face images are
extremely high-dimensional– 100x100 image = 10,000 dimensions– Slow and lots of storage
• But very few 10,000-dimensional vectors are valid face images
• We want to effectively model the subspace of face images
The space of all face images• Eigenface idea: construct a low-dimensional linear
subspace that best explains the variation in the set of face images
Linear subspacesConsider the variation along direction v among all of the orange points:
What unit vector v minimizes var?
What unit vector v maximizes var?
Solution: v1 is eigenvector of A with largest eigenvalue v2 is eigenvector of A with smallest eigenvalue
Principal component analysis (PCA)
• Suppose each data point is N-dimensional– Same procedure applies:
– The eigenvectors of A define a new coordinate system• eigenvector with largest eigenvalue captures the most variation among training vectors x• eigenvector with smallest eigenvalue has least variation
– We can compress the data by only using the top few eigenvectors• corresponds to choosing a “linear subspace”
– represent points on a line, plane, or “hyper-plane”• these eigenvectors are known as the principal components
The space of faces
• An image is a point in a high dimensional space– An N x M image is a point in RNM
– We can define vectors in this space as we did in the 2D case
+=
Dimensionality reduction
• The set of faces is a “subspace” of the set of images– Suppose it is K dimensional– We can find the best subspace using PCA– This is like fitting a “hyper-plane” to the set of faces
• spanned by vectors v1, v2, ..., vK• any face
Eigenfaces• PCA extracts the eigenvectors of A
– Gives a set of vectors v1, v2, v3, ...– Each one of these vectors is a direction in face space
• what do these look like?
Visualization of eigenfacesPrincipal component (eigenvector) uk
μ + 3σkuk
μ – 3σkuk
Projecting onto the eigenfaces• The eigenfaces v1, ..., vK span the space of faces
– A face is converted to eigenface coordinates by
Recognition with eigenfaces• Algorithm
1. Process the image database (set of images with labels)• Run PCA—compute eigenfaces• Calculate the K coefficients for each image
2. Given a new image (to be recognized) x, calculate K coefficients
3. Detect if x is a face
4. If it is a face, who is it?• Find closest labeled face in database• nearest-neighbor in K-dimensional space
Choosing the dimension K
K NMi =
eigenvalues
• How many eigenfaces to use?• Look at the decay of the eigenvalues
– the eigenvalue tells you the amount of variance “in the direction” of that eigenface
– ignore eigenfaces with low variance
PCA
• General dimensionality reduction technique
• Preserves most of variance with a much more compact representation– Lower storage requirements (eigenvectors + a few numbers
per face)– Faster matching
• What are the problems for face recognition?
Enhancing gender
more same original androgynous more opposite
D. Rowland and D. Perrett, “Manipulating Facial Appearance through Shape and Color,” IEEE CG&A,
September 1995 Slide credit: A. Efros
Changing age
•Face becomes “rounder” and “more textured” and “grayer” •original shape
• color both
D. Rowland and D. Perrett, “Manipulating Facial Appearance through Shape and Color,” IEEE CG&A,
September 1995 Slide credit: A. Efros
Which face is more attractive?leftright
Which face is more attractive?
attractive0.5(attractive + average)
Which face is more attractive?leftright
Which face is more attractive?
0.5(adult+child) adult
Limitations• The direction of maximum variance is not
always good for classification
A more discriminative subspace: FLD
• Fisher Linear Discriminants “Fisher Faces”
• PCA preserves maximum variance
• FLD preserves discrimination– Find projection that maximizes scatter between
classes and minimizes scatter within classes
Reference: Eigenfaces vs. Fisherfaces, Belheumer et al., PAMI 1997
Illustration of the Projection
Poor Projection
x1
x2
x1
x2
Using two classes as example:
Good
Comparing with PCA
Variables
• N Sample images: • c classes:
• Average of each class: • Average of all data:
Nxx ,,1
c ,,1
ikx
ki
i xN
1
N
kkxN 1
1
Scatter Matrices
• Scatter of class i: Tikx
iki xxSik
c
iiW SS
1
c
i
TiiiB NS
1
• Within class scatter:
• Between class scatter:
Illustration
2S
1S
BS
21 SSSW
x1
x2Within class scatter
Between class scatter
Mathematical Formulation• After projection
– Between class scatter– Within class scatter
• Objective
• Solution: Generalized Eigenvectors
• Rank of Wopt is limited
– Rank(SB) <= |C|-1
– Rank(SW) <= N-C
kT
k xWy
WSWS BT
B ~
WSWS WT
W ~
WSW
WSW
SS
WW
TB
T
W
Bopt WW
max arg~~
max arg
miwSwS iWiiB ,,1
Recognition with FLD• Use PCA to reduce dimensions to N-C
• Compute within-class and between-class scatter matrices for PCA coefficients
• Solve generalized eigenvector problem
• Project to FLD subspace (c-1 dimensions)
• Classify by nearest neighbor
WSW
WSWW
WT
BT
fld Wmax arg miwSwS iWiiB ,,1
Tikx
iki xxSik
c
iiW SS
1
c
i
TiiiB NS
1
xWx Toptˆ
)pca(XWpca
Note: x in step 2 refers to PCA coef; x in step 4 refers to original data
Results: Eigenface vs. Fisherface
• Variation in Facial Expression, Eyewear, and Lighting
• Input:160 images of 16 people• Train:159 images• Test: 1 image
With glasses
Without glasses
3 Lighting conditions 5
expressions
Reference: Eigenfaces vs. Fisherfaces, Belheumer et al., PAMI 1997
Eigenfaces vs. Fisherfaces
Reference: Eigenfaces vs. Fisherfaces, Belheumer et al., PAMI 1997
Large scale comparison of methods• FRVT 2006 Report
FVRT Challenge• Frontal faces
– FVRT2006 evaluation
False Rejection Rate at False Acceptance Rate = 0.001
FVRT Challenge• Frontal faces
– FVRT2006 evaluation: computers win!
Face recognition by humans
Face recognition by humans: 20 results (2005)
Slides by Jianchao Yang
Things to remember
• PCA is a generally useful dimensionality reduction technique– But not ideal for discrimination
• FLD better for discrimination, though only ideal under Gaussian data assumptions
• Computer face recognition works very well under controlled environments – still room for improvement in general conditions